fix: writable_roots doesn't recognize home directory symbol in non-windows OS (#9193)

Fixes:
```
[sandbox_workspace_write]
writable_roots = ["~/code/"]
```

translates to
```
/Users/ccunningham/.codex/~/code
```
(i.e. the home dir symbol isn't recognized)

codex-rs/utils/absolute-path/Cargo.toml

@@ -9,6 +9,7 @@ license.workspace = true
 workspace = true
 
 [dependencies]
+dirs = { workspace = true }
 path-absolutize = { workspace = true }
 schemars = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
@@ -18,5 +19,6 @@ ts-rs = { workspace = true, features = [
 ] }
 
 [dev-dependencies]
+pretty_assertions = { workspace = true }
 serde_json = { workspace = true }
 tempfile = { workspace = true }